Brushless DC motors are generally required to operate under widely varying conditions of load. These varying conditions include: changes in torque, changes in speed, and resulting changes in power output. Changes in input voltage and commutation angle can be selected to suit the performance capabilities of the brushless DC motor. Of particular interest in respect to the invention to be described are DC motors utilizing permanent magnets, especially of the samarium cobalt type.
Interest in brushless DC motor control has spawned a myriad of technological approaches intended to be responsive to a variety of perceived needs to which the technical solutions have been offered. The invention to be described hereinafter recognizes that motor controls for brushless DC motor systems incorporate what is commonly referred to as open loop phase advance systems. These systems do not operate at optimum efficiency because the phase advance does not change with load demand.
In order to better define the inventive contribution detailed hereinafter, there is now offered a review of the state-of-the-art, beyond which the invention described in this specification will be distinguished.
A study of the prior art reveals that there are a technologically related family of patents which are directed to control circuits for brushless DC motors, which patents recognize the need for and utilize phase comparison circuits to obtain in part a desired motor control.
The prior art that has been uncovered that best describes the background of the invention is exemplified by the patents to Okuyama et al, U.S. Pat. No. 4,088,932 issued May 9, 1978; Alley et al, U.S. Pat. No. 4,250,435 issued February 10, 1981 and Lafuze U.S. Pat. No. 4,295,085 issued October 13, 1981.
Okuyama et al is directed to a control system for a commutatorless motor, in which a large variation in the gain of the current control circuit is prevented, and the speed control loop provided has a high quality of response. The principle feature of the Okuyama et al patent resides in the recognition of the desirability of maintaining the amplitude of the armature current constant when a torque reference signal takes a value smaller than the current intermittent threshold, and at the same time controlling the phase of the armature current in accordance with the torque reference signal so as to maintain the ratio of the output torque of the motor to the commanded or instructed torque substantially constant, independent of the load dependent current.
The invention to be described hereinafter approaches the motor load control in a uniquely different manner in that a real current error is detected and a phase shifted waveform to the motor windings is provided. The phase advanced waveform to the motor effectively corrects the real current error due to load which results in improved motor efficiency.
The Alley et al patent is directed to a motor velocity control system employing feedback to maintain actual motor rotational velocity at or near a desired rotational velocity. The speed control system employs an electronically commutated motor that is adaptable to digital velocity control.
In accordance with Alley et al there is provided a desired motor rotational velocity commanded by an input clock rate, which may be variable, rather than by a voltage or current signal. A sensor is provided for determining actual rotor position, and actual rotor position is periodically compared with instantaneous desired motor position as indicated by the time of occurance of one of the input clock signals or pulses. Depending upon the results of this periodic comparison, a dynamically established active current limit is modified. Specifically, if motor rotor actual position leads desired position, the current limit is reduced. If, on the other hand, actual motor position is lagging, the current limit is increased. If the rotor is in the correct position, the current limit is unchanged.
The invention to be described also employs a sensor to determine actual rotor position. However, the invention of this specification goes beyond a mere current control, in that a real error current error signal is generated that takes into account such factors of real current present in the motor windings, rotor speed and direction, a load position commanded versus an actual load position sensed. The real current error signal of the invention is representative of a phase angle that is proportional to real current error signal.
As will become more clear hereinafter, the detection and the use of real current error signal within the motor provides the key to efficient motor control, especially when load control is a primary objective.
The Lafuze patent is directed to a phase lock loop commutation position control and method for a drive system for a DC field motor having a rotor and a plurality of stator windings. The motor being a permanent magnet motor of the samarium cobalt type, is particularly applicable for use in a variable speed constant frequency (VSCF) motor/generator used in an aircraft environment.
In Lafuze, electric power is supplied to the stator windings of the motor in accordance with gating signals. The gating signals are generated in accordance with timing signals, which are provided in response to a clocking signal. A position signal is furnished as a function of the position of the rotor with respect to a preselected stator position. An error signal is generated proportionally to the phase difference between a selected timing signal and the position signal. The clocking signal is produced as a function of the error signal. In this way, the phase lock loop controls the supplying of the electric power to the stator windings in accordance with the position of the rotor.
In a number of respects the motor and motor winding arrangement of Lafuze is similar to the arrangement provided in the invention described in this specification. By indicating that Lafuze is similar to the invention of this specification it is intended to convey the thought noted at the outset, that Lafuze is like the invention of this specification in that both are directed to control circuits for brushless permanent magnet DC motors of the samarium cobalt type which recognize the need for and utilize phase comparison circuits to obtain in part a desired motor control. The invention of this specification distinguishes over Lafuze in that a real error current signal is provided that is a function of the difference between the magnitude and direction of a real reference current signal derived from an output signal representative of the rotor speed and direction, as well as, a load position command signal and an output signal indicative of a load position sensed. The invention to be described uniquely converts the real current error signal to a locked phase advance waveform signal that is utilized to control the motor in a load responsive manner.
In the prior art patents described above, the teachings were intended to convey the state of the relevant motor control art. In the invention to be described there is also provided a phase advance waveform generator that receives the real error current error signal mentioned, and provides a controlled locked phase advance waveform signal to control a DC power supply that furnishes power to the motor to be controlled. This waveform generator takes the form of a loop and uniquely performs and cooperates with the motor control circuit of the invention.
Two patents have been uncovered that, when once described, will afford the reader a basis for appreciating the inventive advance in the loop to be described in respect of the waveform generator.
The first of which is the patent to Konishi et al, U.S. Pat. No. 3,668,492 issued June 6, 1972, and the second, U.S. Pat. No. 3,887,820 issued June 3, 1975 to Glennon, one of the co-inventors of this invention.
The Konishi et al patent is directed to a servo system for controlling a motor. There is provided a servo loop that includes a phase comparator for providing an error signal representative of the phase difference between an input reference periodic signal and an oscillatory signal for driving a motor.
A voltage controlled oscillator controlled by the error signal to produce the oscillatory signal is provided with an integrator for integrating the error signal to produce an integration signal proportional to the result of the integration of the error signal and therefore to hold the instantaneous value of the integration signal when the error signal becomes zero. The integration signal is supplied to the oscillator in place of the error signal.
The controlled locked phase advance loop to be described hereinafter clearly departs from that suggested by Konishi et al. There is nothing in Konishi et al that would motivate someone skilled in the art to add a signal summing circuit between the phase comparator 14 and the integrator 22 to introduce an error signal that represents a desired phase shift in waveform of the signal to drive the motor. The absence of a summing circuit of the nature and at the location just described, as well as the failure to appreciate the significance of real current error renders the Konishi et al patent devoid of an anticipatory teaching of the control system of the invention.
The Glennon patent and its operation should be understood because such an understanding will enhance the readers awareness of the substantial departure the subject invention provides over the earlier fine piece of intellectual property.
Glennon is directed to paralleling control of phase synchronized generators, and teaches the use of a phase comparator and an integrator in a generator control system. The phase of the generator is compared with the phase of the reference signal and the output of the phase comparator is connected with a summing integrator, the output of which is a DC signal with an amplitude representing the cumulative angle of phase difference between the generator output and the reference and the polarity indicating whether the generator output leads or lags. The signal at the output of the summing integrator is used to control the variable speed drive for the generator. Only when the generator and reference are in substantial phase synchronism is the switch connecting the generator output in parallel with another source closed.
In the invention to be described there is provided a signal summing circuit between the phase comparator and integrator, which summing circuit allows an error signal to be introduced, which error signal produces a phase shifted waveform. Glennon's primary motive is to obtain control alternating signals in substantial phase synchronism while the invention to be described desires a controlled locked phase advance waveform signal.